Powder Technology 191 (2009) 155–163

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Fabrication of ultrafine powder from eri silk through attritor and jet milling

Rangam Rajkhowa a, Lijing Wang a, Jagat Kanwar b, Xungai Wang a,⁎a Centre for Material and Fibre Innovation, Deakin University, Geelong, VIC 3217, Australiab Institute of Biotechnology, Deakin University, Geelong, VIC 3217, Australia

⁎ Corresponding author. Tel.: +61 3 522 72894; fax: +E-mail address: xwang@deakin.edu.au (X. Wang).

0032-5910/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.powtec.2008.10.004

a b s t r a c t

a r t i c l e i n f o

Article history:

Fibroin protein derived from Received 9 April 2008Received in revised form 21 August 2008Accepted 1 October 2008Available online 17 October 2008

Keywords:Silk powderMillingParticle sizeEri silk

silk fibres has been extensively studied with exciting outcomes for a number ofpotential advanced biomaterial applications. However, one of the major challenges in applications lies inengineering fibroin into a desired form using a convenient production technology. In this paper, fabrication ofultrafine powder from eri silk is reported. The silk cocoons were degummed and the extracted silk fibreswere then chopped into snippets prior to attritor and air jet milling. Effects of process control agents,material load and material to water ratio during attritor milling were studied. Compared to dry and dry–wetattritor milling, wet process emerged as the preferred option as it caused less colour change and facilitatedeasy handling. Ultrafine silk powder with a volume based particle size d(0.5) of around 700 nm could beprepared following the sequence of chopping ➔ wet attritor milling ➔ spray drying ➔ air jet milling. Unlikemost reported powder production methods, this method could fabricate silk particles in a short time withoutany pre-treatment on degummed fibre. Moreover, the size range obtained is much smaller than thatpreviously produced using standard milling devices. Reduction in fibre tenacity either shortened the millingtime even further or helped bypassing media milling to produce fine powder directly through jet milling.However, such reduction in fibre strength did not help in increasing the ultimate particle fineness. The studyalso revealed that particle density and particle morphology could be manipulated through appropriatechanges in the degumming process.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Silk fibroin macromolecules have been extensively studied inrecent years as biomaterial for a number of different non textileapplications. This has been done through re-fabrication of native silkfibres as nano fibres [1–3], films [2,4], hydrogels [5,6], foams [2,7],coating materials [8] and powders. Among these forms, silk powder isnow produced commercially as an additive in cosmetic products andfunctional foods. Other potential applications of silk powder such assurface coating [9–11], fibre treatment [12,13], fillers in films [14], ink[15] wound care [16], enzyme immobilisation [17–19], compositescaffold for cell growth [20] and drug delivery [21,22] have also beenreported.

Powder can be prepared either by a solution route or byamechanicalmethod. In the solution route, silk fibres are dissolved first in aconcentrated chaotropic salts followed by removal of the salts, and thenpowder from the aqueous silk solution is regenerated through a numberof different means [17,23–25]. The mechanical method, on the otherhand, avoids lengthy, costly and environmentally sensitive productionprocedures associated with the solution route. It also retains thecrystallinity and water insolubility of fibroin particles [26]. The present

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study adopted the mechanical attrition of fibres to fabricate ultrafineparticles.

Unlike inorganic materials such as metals and ceramics, organicpolymermaterials are very difficult tomill into fine particles. Milling ofsome synthetic polymers has been reported in the literature [27–29].Early works on silk milling used mostly ball and jet milling [14,26].Long milling time (up to 20–40 h) and restricted fineness (d(0.5)=10−20 µm) remain themajor issues inmechanical silkmilling processes. Toovercome these, various pre-treatments were used to reduce fibrestrength [26,30–32]. Such an approach was also used for fabricatingwool powder [33]. While the pre-treatments could increase millingefficiency, they were not effective in reducing particle size to sub-micron scale. For some applications, sub-micron particles are likely tohave advantages as they are highly reactive due to the high surface-to-volume (or mass) ratio [21,23,26,34]. Availability of such ultrafinepowder may also open out new application avenues of powdered silk.A method to prepare nano scale silk particles was reported [35],however, it relied on a special device using a high pressure of 1000 kgf/cm2. The process also filtered large particles and used number baseddistribution to calculate the particle size, which is normally muchsmaller than the volume based calculation. Fabrication of sub-micronpowder through standard milling devices is yet to be realised and wehave made good progress in this direction in our recent study [36].

In the previous study, we discussed the silk fragmentation mecha-nism and milling kinetics using rotary and planatary ball milling [36].

Fig. 1. Schematic diagram of milling sequence.

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We obtained sub-micron particles through wet planatary ball millingusing fine media, but subsequent drying caused particle agglomera-tion. In that study, the importance of friction in separating micro-fibrillar architecture of silk was demonstrated through fibre bendingabrasion fatigue results. Use of process control agent (PCA) at the startof ball milling hindered the silk milling process as it reduced thefrictional forces during grinding. We also observed that powderproduction remained inefficient without reducing fibre tenacity to acertain extent via intensive degumming. Silk degumming is a chemicalprocess that removes the glue like protein that surrounds the silkfilaments and holds two individual filaments together.

Preserving the native fibre composition during powder productioncould be important for some applications. As an example, degradationduring degumming sharply reduced performance of silk fibroinprotein to the attachment and growth of cells in tissue cultureexperiments [37]. During the degumming process, some degradationand associated decrease in fibre strength are unavoidable [38,39].However, most of this strength can be retained following a standarddegumming procedure used in this study.

In the present study, an attritor mill was used to prepare ultrafinepowders from eri silk fibre produced by Philosomia cynthia ricinisilkworm. Unlike rotating container in a planetary ball mill used in ourprevious study, in the attritor, the balls are stirred inside a stationarycontainer. This allows more irregular movement and spin to themedia, resulting in a higher shear force and more frequent particle/media collision than is the case for ball milling [40]. The use of attritorfor ultrafine and nano scale particle production and dispersion hasbeen reported in recent years [41–44]. Most of thematerials processedon the attritor mill are, however, inorganic in nature and its suitabilityfor soft organic materials such as silk fibre has not been examined. Inthis study, air jet milling was also employed to assist furtherrefinement of the attritor milled particles/aggregates. Instead ofusing a grinding media, air jet mill uses the kinetic energy of thepressurised air, and comminution takes place due to impact betweenparticles. Direct milling of silk snippets to powder using the jet millonly was also examined.

2. Experimental procedures

2.1. Material preparation

Eri silk cocoons, which were open mouthed and free from pupae,were purchased from Northeastern India. They were degummed witha laboratory dyeing machine (from Thies) using laboratory gradesodium carbonate 2 g/L and sodium dodecyl sulphate (from SigmaAldrich) 0.6 g/L at 100 °C with a material (g) to liquor (L) ratio 1:25 for120 min. The batch size of cocoons was 1 kg. To reduce fibre strengthfor comparison trials, sodium carbonate concentration was increasedto 10 g/L and temperaturewas raised up to 120 °C in some degummingbatches.

2.2. Powder production devices

2.2.1. Pre-millingA cutter mill (Pulverisette 19 from Fritsch) was used for chopping

degummed silk. It has five “V” shaped blades fitted on a rotor, whichrotates at a set speed of 2888 rpm. These blades in combination withthree fixed blades cut the fibres until they became fine enough to passthrough a 1 mm grid. In other words, this process reduced silkfilaments to approximately 1 mm snippets. Some snippets longer than1 mm could also pass through the grid due to the small diameter ofsilk fibres (10–20 µm).

2.2.2. Attritor millingAttritor (1S fromUnion Process) with a 9.5 L tankwas used for both

dry andwet grinding. Yttrium treated zirconium oxide grindingmedia

(5 mm) weighing 20 kg and with a bulk volume of 5.5 L was used forall experiments. Stirrer was set at 280 rpm. Cooling water (approxi-mately 18 °C) was circulated to minimise silk thermal degradationduring attritor milling.

2.2.3. Air jet millingSturtevant laboratoryair jetmillwith grinding airpressure110kg/cm2

was used.Materialwas fed using a powder hopper (K-Tron,made inUSA)at a feed rate of 200 g/h.

2.2.4. Spray dryingDry powders from wet milled slurry were recovered through a

laboratory spray dryer (B-290 from Buchi Labortechnik AG). The inlettemperature and slurry flow rate were 140 °C and 7–8 mL/min,respectively.

2.3. Particle production

Fig. 1 shows the schematic diagram of themilling sequence used inthis study. Degummed silk was chopped into snippets for further sizereduction. The snippets were ground either dry or with water in theattritor. In some wet milling experiments, snippets were pre-drymilled before wet milling. Powder from milled slurry was recoveredusing the spray dryer. Air jet milling was the final step in powderproduction. Some snippets were also directly passed though the air jetmill without going through the media milling process.

Preliminary dry and wet grinding trials were performed in order todetermine the optimum loading of snippets in the attritor. Accord-ingly, unless specified otherwise, the snippet loading in all experi-ments was kept constant at 200 g standard mass per batch. Duringeach experiment, if not specified otherwise, powder/slurry samples(b50 mg) were collected at an interval of 1 h and particle size wasmeasured.

To reduce static charge during dry milling, 1% antistatic agent(ANLWS) based on weight of snippets was mixed with water (20:1water to antistatic agent ratio) and sprayed over the snippets. Materialwas conditioned before milling at room temperature for at least 24 hto dry the sprayed water. Also, 1–5% (on weight of snippets) stearicacid (from Sigma Aldrich) or polyethylene glycol (PEG, MW 10,000,from Fluka) was added gradually either at the start or after a certaintime (1, 2, 4 h) of milling as the process control agents (PCA) duringdry milling. Stearic acid is commonly used as PCA to improve particlesize reduction in metal powder processing. PEG is a biocompatible

Fig. 2. Volume d(0.5) vs. time of milling of eri silk snippets (dry milling).

Table 1Effect of % of PCA on particle size reduction (PCA added at start)

d(0.5) in µm after 1 h d(0.5) in µm after 2 h

1% Stearic acid 21.4 11.53% Stearic acid 14.5 11.65% Stearic acid 13.3 11.03% Polyethylene glycol 14.9 10.3

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polymer and is also used as a PCA in inorganic material milling. It wastherefore selected in view of the possibility of using silk powder forbiological applications. Neither antistatic oil nor PCA was used whenthe material was dry–wet and wet milled. During wet milling,snippets/powder (g) to water (mL) ratio was 1:2.5–1:15.

2.4. Particle size measurement

2.4.1. Laser particle analyserMalvern Instruments Mastersizer 2000, which is a laser diffraction

based particle size analyser, was used to measure particle size. Hydro2000S side feeder was used to disperse particles for measurement.The dispersion medium was Propan-2-ol (from Sigma-Aldrich). Thesoftware (version 5.21) that came with Mastersizer 2000 was used foroperation and data analyses. The software uses a least squares methodwith smoothness constraint (Tikhonov regularization) to solve theequations that connect size distribution with scattering energy. Thishelps regularisation and reduction of noise. The fundamental result ofcumulative volume below a set of size intervals is fitted by thesoftwarewith a set of piece-wise cubic splines. Interpolation using thesplines gives the ‘frequency’ curve andmedian size. Refractive index of1.542 and imaginary refractive index of 0.01 for eri silk were used fornecessary calculations by the software. All particle size resultsreported are based on volume distribution of particles. As the powdersamples produced are homogeneous, the variation between measure-ments within a batch is insignificant. Hence no error bars are plottedin d(0.5) measurements in this paper.

2.4.2. SEMMorphology of gold sputter coated particles was observed under a

scanning electronmicroscope (LEO 1530 FEG-SEM) at 2 kV acceleratedvoltage and 2–4 mm working distance. SEM images of fibre cross-sections were also obtained to determine the fibre linear density.

2.5. Measurement of fibre tenacity

Single fibre tenacity was measured to determine the level ofdamage caused by degumming. A Single Fibre Analyser (SIFAN fromBSC Electronics) was employed to measure fibre breaking load using ajaw separation speed of 500 mm/min and a gauge length of 25 mm.The tested fibres were sampled from the middle degummed cocoonlayers. Their linear density, in dtex or gram per 10,000 m, wascalculated from the measured mean cross-sectional area of cutfilaments collected from the same cocoon layers as used for tensiletests. Fibre tenacity was calculated from the fibre breaking load and itslinear density. A total of 100 silk fibres were tested from each de-gummed batch.

2.6. Measurement of colour change

DIN6167 Yellowness and Ganz Grieoser Whiteness indices ofpowders were measured using a spectraphotometer SF600 Plus-CT(from Datacolor). Powders were compressed at a pressure of 2000 psiin a cylinder for 5 min to form 16 mm diameter discs. The discs wereused for colour measurement.

2.7. Measurement of tapped density

Powder samples were gradually filled in a Teflon cylindermeasuring 30 mm in diameter and 28 mm in height. The cylinderwas tapped gently while filling the powder to allow the powder tosettle uniformly. The cylinder top was levelled with a glass slidewithout applying any downward pressure. The weight of the powderwas then measured and the density was calculated.

Before measuring fibre tenacity and powder tapped density, allsampleswere conditioned at 20±2 °C and 65±2% relative humidity for48 h.

3. Results and discussion

3.1. Dry attritor milling

Fig. 2 shows particle size change during dry attritor milling and theinfluence of PCA was added at different times of milling. It took only2 h when the PCA was added at the early stage of milling or 4 hwithout PCA to fabricate powders with a volume d(0.5) of about10 µm. The milling time reduction is significant compared to 12–18 hrequired in an earlier study using planetary ball milling [36]. Thisdifference may be attributed to the higher shear and impact energyprovided by the attritor to fibrillate and fracture particles. It is knownthat a laboratory attritor works much more efficiently than aconventional ball mill [44]. The result of the present study provedthe same for silk milling. The result also reveals a different role of PCAin attritor milling compared to planatary ball milling. Previously, weobserved that 3% stearic acid as PCA had a negative effect when addedat the start of silk dry milling because of the reduction in frictionwhich impaired fibrillation of snippets [36]. In attritor milling, resultsshow improvement in milling performance with PCA whether it wasadded at the start or duringmilling, though less effective if added after4 h dry milling. This result indicates that there is sufficient frictionavailable in spite of the presence of PCA in the attritor milling foreffective size reduction of silk. PCA may assist the process bypreventing aggregation of small particles. However, it can be seenthat PCA does not help much in increasing the limiting fineness ofparticles. Effectiveness of PCA is reduced as particle size gets finer.There is also a possibility of PCA decomposition with time, which hasbeen reported to occur during mechanical alloying of metal powderthrough ball milling [42,43]. The influence of the quantity of PCA indry milling was also studied. As seen from Table 1, adding stearic acidbeyond 3% had only marginal benefit. Considering possible contam-ination introduced by PCA,1–3% addition of PCA appears to be a betterchoice. Polyethylene glycol which is biocompatible, may be effectivelyused for silk milling.

It was observed that strong static electricity was generated duringmilling and as a result the milling media was covered by a thick layer

Table 2Colour results of attritor milled eri silk powder

Material type Whiteness index (Ganz Grieoser) Yellowness index

Dry milled 6 h 55.8 22.4Wet milled 6 h 71.9 12.8

Fig. 4. Fibre tenacity vs. wet milling time to fabricate particles with volume d(0.5) ofaround 5 µm.

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of powder. It caused problems in separating ground powder from themedia and collecting it from the milling container. The staticelectricity was due to the synthetic lining inside the cylinder walland zirconium oxide sleeved agitating arm of the attritor, whichprovides an electricity insulating environment. In addition, abradingpoor electrically conducting silk material during milling compoundedthe problem. Therefore, 1% antistatic oil was sprayed on the snippetsto reduce the static problem. It was observed that PCA also helped tostrip off powder from themedia, but only up to a limit while it remainseffective. Apart from the static build-up during milling, longer millingtime increased the propensity of particles sticking to themedia as theybecame finer. Strong adhesion to media influences the movement ofparticles resulting in a reduction of the milling efficiency and powderyield. In addition to static generation, poor thermal conductivity offibre material is also detrimental to dissipation of local heat generatedduring drymilling. Circulated coolingwater through the attritor jacketthough reduced the overall temperature inside the vessel to someextent, local heating probably caused some thermal degradation ofpowder material. It is clear from Table 2 that compared to wet milling,dry milling produces dull and yellowish powder.

3.2. Wet attritor milling

3.2.1. Wet milling of snippetsExperiments were carried out to study milling kinetics and the

optimummilling conditions in awetmilling process using silk snippets.

3.2.1.1. Snippets to water ratio. To optimise snippets to water ratio,tests were performed in the range 1:2.5 to 1:15. This rangewas selectedbased on some preliminary studies. A low ratio produces a thick slurry,which is difficult to drain out from the attritor and to subsequently spraydry. On the other hand, excessive water increases the volume, resultingin spilling out liquor from the system. The snippets to water ratio waschanged by changing the quantity of water while keeping the snippetsload constant at 200 g and the amount ofmedia constant at 20 kg. Fig. 3reveals that as the snippets to water ratio increases, the millingefficiency improves first, but then drops. Results in this study agreewith the general trend reported with respect to solid content in theslurry during milling of some inorganic materials [41–43]. It is reportedthat higher solid content leads to increased slurry viscosity whichreduces kinetic energy of grindingmedia and as a result a lower millingrate is achieved.On the other hand,when it is diluted, thoughviscosity isreduced, collision frequency of particles drops as less particles arecaptured in the active grinding zonebetween the grindingbeads. A good

Fig. 3. Effect of snippets to water ratio on particle size reduction in wet milling.

balance of collision intensity and collision frequency is required for goodmilling performance. For eri silk milling, the best results could beobtained using around 1:7.5 snippets towater ratiounder the conditionsexamined.

3.2.1.2. Degumming condition. The tenacity of silk fibre degummedthrough the applied normal degumming condition was 3.6 cN/dtex.The tenacity was drastically reduced with an increase in chemicalconcentration and/or temperature. The change in fibre tensilestrength has a significant impact on the milling performance, asdemonstrated in Fig. 4. Milling time to reach particle volume d(0.5)around 5 µmdrops at a linear rate with the drop in the fibre tenacity asindicated from the three test results of milling conducted with fibresof different tenacities. The decrease in milling time however does notensure an increase in the limiting fineness of particles. In fact, particlesize still stabilized at around 5 µm and remains independent to fibretenacity during wet milling. This probably happened due to particleaggregation and/or resistence to further fracture for the 5 mmgrinding media used in these experiments.

3.2.1.3. Material loading. Silk snippets form a pulp when mixed withwater. Owing to the thorough mixing and simultaneous reduction inparticle size during the course of milling, large fibre particles areground into small ones, and the mixture becomes a homogeneousslurry. The time required to reach the slurry state varies dependingupon the silk degumming conditions and material loading. A highmaterial loading in the attritor will slow the media movement and theintensity of milling. The material mixure and the media movementcan be observed through the feeding port of the attritor during the

Fig. 5. Effect of snippets loading and degumming condition on wet milling efficiency.

Fig. 6. Volume d(0.5) vs. time of wet and dry–wet milling of normally degummed erisilk.

Fig. 8. Change of volume d(0.5) after spray drying.

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grinding process. Material state can also be observed by sampling andevaluating the product through the output port from time to time.

As shown in Fig. 5, for normally degummed silk and 400 g snippetsloading, the drop in d(0.5) in the first 6 h was rather slow. The mixurestill looked like a pulp, whereas after 12 h milling, the sample turnedinto a slurry. However, when reducing both snippets and wateramounts by half without changing the grinding media, the materialturned into a slurry during the first hour. The dramatic change inmilling efficiency is reflected in Fig. 5. Particles with d(0.5) around10 µm could be fabricated in 2–3 h. Hence, 200 g snippet loadwas usedto carry out the rest of experiments using normally degummed silk.

On the other hand, no such difference was observed whilechanging the snippets loading from 400 to 200 g when the fibrestrength was reduced from 3.6 cN/dtex to 0.8 cN/dtex by changing thedegumming conditions. As seen in Fig. 5, the plots for 200 and 400 gloadings almost overlap for such intensively degummed low tenacityfibre snippets. Though the snippet size was around 42 µm comparedto 63 µm of normally degummed fibres, the rate of size reduction forintensively degummed snippets was significantly different and even400 g loading could be milled rather easily.

3.2.2. Dry–wet millingAs discussed earlier, dry milling caused a strong particle adhesion

to the milling media which made collection of powder difficult aftermilling. Adhesion perhaps also contributes towards aggregation ofparticles effecting particle size reduction. A combined dry–wet processmay alleviate this problem. In silk planatary ball milling, the dry–wetroute achieved good milling results on low tenacity fibres [36]. Thiscombined process was therefore employed in this attritor milling to

Fig. 7. Particle size distribution of dry–wet milled slurry.

improve particle fineness prepared from regular tenacity silk. Asshown in Fig. 6, the combined process reduced particle size furthercompared to wet milling only. Bimodal particle size distribution inFig. 7 clearly demonstrates the presence of sizeable amount of sub-micron scale particleswhen the dry–wet combinedmilling progressedto 7 h. Continued milling up to 11 h, however, resulted in the aggre-gation of the fine particles represented by the corresponding singlemode distribution plot in Fig. 7. An associated increase in volume d(0.5)at that time is also reflected in Fig. 6. To overcome aggregation, slurrywas diluted by increasing slurry volume by 50% with the addition ofwater and further milled for an hour. The return of the bimodaldistribution is possibly due to particle de-aggregation. Though theprocess has an advantage over wet milling in improving particlefineness, the dry milling phase caused significant colour change.

3.3. Spray dried attritor milled particles

3.3.1. Normal tenacity silkFig. 8 presents the change in d(0.5) during spray drying process. It

can be seen that particle size does not changemuch upon dryingwhend(0.5) is approximately 5 µm or larger. However, a difference isnoticed on spray drying finer particles. Sub-micron scale particles ifpresent in the slurry disappear after spray drying. An example fromslurry preparedwith single stepwetmilling of snippets is presented inFig. 9. This indicates aggregation of fine particles in the dry powder.

SEM images presented in Fig.10 shed some light on the aggregationprocess. Silk fibres are made of microfibrils and the mechanical forcesduring milling can overcome the cohesive forces between microfibrilsand separate them [36]. The fragments of separated fibrils tend toaggregate into spheres. The 3 hmilled SEM image in Fig. 10 shows thataggregates are made up of fibrous particles. As wet milling continues,dynamics of fragmentation, aggregation and de-aggregation yields a

Fig. 9. Particle size distribution of wet attritor milled silk particles.

Fig. 10. SEM images of spray dried eri silk particles after wet milling.

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steady state with no further change in particle size. During this steadystate, the irregular aggregates slowly become more spherical andcompact as shown in the SEM image (Fig. 10) of 12 h milled particles.The size of a large number of primary particles in the aggregates is lessthan 1 µm. Though these primary particles remain in aggregated formafter spary drying, some of them are actually in a dispersed state in theslurry as demonstrated in the bimodal distribution in Fig. 9. Theseparticles as they aggregate neither appear in the paticle sizedistribution after spray drying in Fig. 9 nor can be observed in theSEM image of dried powder in Fig. 10.

Fig. 11. Eri silk snippet (intensively degummed).

Fig. 12. SEM images of eri silk particles (intensively degummed).

While it is clear from Figs. 9 and 10 that a large number of sub-micron particles were produced in the milling process, it is not clearwhether those in the range1–10 µmshown in the bimodal distribution(Fig. 9) are made up of only aggregates of fine particles or representlarge primary particles with globular surface texture. Silkworm silk aswell as spider silk form fine globular microsctructure with 100–200 nm globes at the broken tips during tensile fracture and the globesare confirmed asmicrofibrils of silk [45]. A similar observationwas alsoreported during pulling of silk fibres under liquid nitrogen [46]. Suchglobular microstructure of fractured silk is attributed to the reversionof extended molecular chains when a force is applied to execute afracture. During milling, fibre particles are subjected to forces of

Fig. 13. SEM image of eri silk (intensively degummed) directly air jet milled fromsnippets.

Table 3Tapped density of spray dried particles

Wet milling time Tenacity of degummed fibresused (cN/dtex)

Tapped density (g/cm3) of spraydried particles

15 min 0.8 0.503 h 0.8 0.553 h 3.6 0.28

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different nature, including shear forces. It is possible that during thefracturing process, themicrofibrils roll up and form a globular texturedsilk particle from a single silk fibre. The globular surface texture isshown in themagnified portion of the silk snippet in Fig.11. That part ofthe fibrewas subjected to some abrasion during the chopping process.

3.3.2. Low tenacity silkSize reduction rate on intensively degummed snippets during

attritor milling was very fast as reflected in Fig. 5. Particles with d(0.5)less than 10 µm could be fabricated within 15 min milling (Figs. 5 and12). Considering the diameter of eri silk which is around 15 µm, it isclear that after 15 min milling, particles and fragments of splittedfibres with globular microstructues are formed. As milling continuedup to 30 min and beyond, the remaining fibrous particles werefragmented and then no obvious comminution took place. It is knownthat as particles become smaller further comminution is moredifficult. However, the texture of particles becomes smoothenedwith continued milling. Such a change is probably due to thedominance of plastic behaviour of the intensively degummed silkmaterial. The 3 h milled particles apparentlly look solid due to suchsurface smoothening process. The increased compactness is reflectedin the results of tapped density measurements shown in Table 3. Thetapped density of normal degummed fibre particles (with a highertenacity) is much lower that that from the intensively degummed silk(with a lower tenacity), in spite of the increased milling time. Themarked difference in particle size, shape and texture between silkpowders prepared from normally degummed and intensivelydegummed fibres is therefore related to the changes in viscoelasticbehaviour of particles caused by a change in the degumming intensity.

3.4. Air jet milling

3.4.1. Air jet milling on snippetsTable 4 shows that air jet milling on snippets is effective, if the fibre

strength is reduced by changing the degumming conditions. Themolecular weight of silk is sharply reduced during a harsh degum-ming treatment [47]; accordingly a marked decrease in tensile andabrasion resistence can be observed after such a treatment [36]. Silkfibre morphology consists of microfibrils assembled in the form ofpleated sheets [45,48]. Due to such morphology, separation of thesesheets starts on the application of mechanical forces [49]. It happensmuch faster after severe reduction in fibre strength [36]. Even during

Table 4Change in particle size on air jet milling

Wet attritormilling time

Type ofdegumming

Volume d(0.5) in µm

Before air jet milling After air jet milling

0 (snippets) Intensive 42.5 4.315 min Intensive 9.8 5.330 min Intensive 7.4 5.61 h Intensive 6.6 6.13 h Intensive 6.8 6.30 (snippets) Normal 63 613 h Normal 8.9 1.76 h Normal 5.6 0.712 h Normal 5.6 0.8

the chopping process, some fragments were formed (Fig. 11). Thecollision energy provided by jet milling could separate microfibrillarsheets and produce fine powder once the fibre strength was sharplyreduced. As there is no media in air jet milling to help congregatethese fragmented sheets into other shapes, ultimate particlesremained flaky in nature when jet milling was used directly on silksnippets. This is seen in the SEM image in Fig. 13. Absence of globularmicrostructure on the surface of these particles in contrast to mediamilled particles may be due to the lack of friction in air jet millingprocess. The direct air jet milling could be the preferred way ofproducing fine silk powder if changes in material resulting from harshdegumming conditions are acceptable. Apart frommaking the processshorter, avoiding the use of attritor eliminates any possible risk ofcontamination from the milling media as it is known that contamina-tion may not be totally avoidable in media milling [44].

3.4.2. Air jet milling of spray dried particlesAggregation in slurry during the milling process has been already

discussed. One way of overcoming this and reducing silk powder sizeis the use of finer media. Using fine media for preparing nanodispersion of inorganic materials has been reported [43]. Our previousexperiment also revealed better performance of finer media on silkmilling [36]. However, finer media also increase the risk ofcontamination. Moreover, although particles are reduced to sub-micron scale during wet milling, it is difficult to obtain such sub-micron particles in dry form. Air jet milling was therefore used to

Fig. 14. Particle size distribution of air jet milled silk particles.

Fig. 16. SEM image of intensively degummed eri silk particles: (a) 15 min attritor milledthen air jet milled and (b) 3 h attritor milled then air jet milled.

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explore the possibility of de-aggregation and fragmentation of spraydried particles to generate ultrafine particles in dry form.

It can be seen from Table 4 as well as in the distribution plot of jetmilled particles in Fig. 14 that air jet milling can reduce particles ofnormally degummed fibres significantly and d(0.5) less than 1 µm canbe achieved. SEM image of such fine particles is provided in Fig. 15.Comparing this with SEM image before air jet milling in Fig. 10, itappears that air jet milling largely separates primary particles fromthe aggregates. However, jet milling could not provide a unimodalparticle size distribution below 1 µm as reflected in the plot in Fig. 14.Some particles more than 1 µm are visible also in Fig. 15. So thequestion remains as to whether or not the aggregates shown in Fig. 10contained some large size primary particles.

The result of air jet milling of intensively degummed fibre premilled by attritor is however diffierent from normally degummedfibre particles. It is seen from Table 4 that air jet milling failed to havenoticeable impact on particle size reduction of intensively degummedand attritor milled silk particles. This is further confirmed bycomparing SEM images between Figs. 12 and 16. Air jet millingworked well on fibrous shaped particles, but was not effective onattritor milled fine powder. The attritor milled particles were verycompact and so could not be easily fractured. These results clearlyindicate that a substantial reduction in fibre tenacity provides a scopeto cut downmilling time significantly. It also brings changes to particleshape and powder bulk density. At the same time, too low fibretenacity stands in the way of fabricating ultrafine sub-micron scaleparticles.

4. Conclusions

Fine silk powder can be fabricated in sizeable quantity usingsuitable standard grinding devices from degummed fibres withoutany pre-treatment. Dry attritor milling was not a preferred approach,due to high static charges resulting in powders sticking to millingmedia, which reduce powder yield. From that consideration, wetmilling following the dry milling was a better option. The combinedprocess produced finer particles compared to single dry or wet attritorprocess alone. To avoid possible colour change during dry milling, wetmilling only was eventually considered as the best option. By using5 mm grinding media and appropriate amounts of material andmaterial to water ratio, silk particles having a volume based particlesize d(0.5) in the range 5–10 µm can be fabricated within 3–6 h withwet attritor milling. During wet milling, fibrous shaped silk snippetsturned into irregular sub-micron particles with time, but sphericalaggregates of these primary particles were distinct, particularly afterspray drying. Under the conditions examined, such aggregation was

Fig. 15. SEM image of eri silk particles: 6 h attritor milled then air jet milled.

responsible for stabilising the d(0.5) around 5 µm in spite of longermilling time. Subsequent use of air jet milling to free the ultrafineprimary particles from the aggregates proved useful. The ultimateparicles with d(0.5) of approximately 700 nm could be produced fromfibres without harsh degumming or any other pre-treatments. Fibretenacity played an important role in milling efficiency and particlemorphology. Low tenacity snippets resulted in a short milling timeand a linear relationship between milling efficiency and fibre tenacitywas recorded. With a substantial reduction in fibre tenacity, highlycompact particles were formed, and air jet milling was not effective tobreak these further into sub-micron primary particles. The study alsorevealed that, unlike normally degummed silk fibres where mediamilling was necessary, low strength fibre snippets could be directlyconverted by air jet milling into flaky particles with a volume basedparticle size d(0.5) of around 5 µm.

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